A Dragon Ally?

[Andor]

Here's my take. Treat it as you would a Mount. Deduct the Dragons xp value from the encounter as you would if he were a mount.

That's probably the best way to do it (obviously the encounters are more difficult as well and worth abit more xp)

What he said. If you look in the DMG under mounts on page 46 you'll see that for 'big guys' like the Dragon they suggest adding his XP value to the encounter and then not awarding it to the PCs making him sort of 'value neutral.'

Alternately he could be so much trouble in combat that the pc feel the need to slow him down out of self preservation in which case treat him like a normal mount and make some PC ride him and split his actions between the pc and the dragon.

 

[Skyscraper]

What I'm planning on doing for the moment is this: Give them one fight where the dragon fully participates,and scale the xp encounter for it. We're talking about a devil army with two war devils, a pit fiend, and a hired on balor and elementals. The dragon draws the fire of the elementals and the balors, leaving a manageable fight for the PCs. The dragon decides after that (understandibly) that this is just too much and won't fight with them anymore, flying off to discover what else there is to do in the world. It then comes back for the last fight, following them all the way into the Abyss to help them defeat Orcus, Prince of the Undead himself. At the cost of his life, of course.

How about simply playing the dragon along and seeing how it goes? In your first post you said you didn't want to take the dragon away from the players, yet here you say that after one fight he'll leave to come back for the finale.

You can do what you feel like doing of course; i'd simply recommend seeing how things develop before planning to remove the dragon from the story.

Sky